U.S. patent application number 11/447500 was filed with the patent office on 2006-12-21 for tracking system with excited state atomic line filter.
Invention is credited to Michael Joseph Lefebure, Daniel H. Leslie, Zhong-Quan Zhao.
Application Number | 20060285569 11/447500 |
Document ID | / |
Family ID | 46324609 |
Filed Date | 2006-12-21 |
United States Patent
Application |
20060285569 |
Kind Code |
A1 |
Zhao; Zhong-Quan ; et
al. |
December 21, 2006 |
Tracking system with excited state atomic line filter
Abstract
A tracking system utilizing an excited state atomic line filter.
The filter includes a metal vapor cell having an optical entrance
port and an optical exit port and containing a metal vapor having a
first excited energy state with a resonant frequency, and a second
excited energy state. The cell has an absorption line, at or near a
desired filter wavelength. The platform to be tracked, which could
be an un-manned aerial vehicle has a beacon laser system located on
it for producing a beacon laser beam at a wavelength within the
narrow spectral band. The present invention solves the problem of
lack of ground state resonant lines in at wavelengths substantially
longer than those of visible light. Atomic line filters of the
Faraday or Voigt crossed polarizer type are provided in which
alkali metal atomic vapor in a vapor cell is excited with a pump
beam to an intermediate excited state where a resonant absorption
line, at a desired wavelength, is available. A magnetic field is
applied to the cell producing a polarization rotation for polarized
light at wavelengths near the resonant absorption lines. Thus, all
light is blocked by the cross polarizers except light near one of
the spaced apart resonant lines. However, the polarization of light
at certain wavelengths near the resonant is rotated in the cell and
therefore passes through the output polarizer.
Inventors: |
Zhao; Zhong-Quan; (San
Diego, CA) ; Leslie; Daniel H.; (Encinitas, CA)
; Lefebure; Michael Joseph; (Del Mar, CA) |
Correspondence
Address: |
TREX ENTERPRISES CORP.
10455 PACIFIC COURT
SAN DIEGO
CA
92121
US
|
Family ID: |
46324609 |
Appl. No.: |
11/447500 |
Filed: |
June 5, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10682567 |
Oct 9, 2003 |
7058110 |
|
|
11447500 |
Jun 5, 2006 |
|
|
|
Current U.S.
Class: |
372/56 ;
250/201.1 |
Current CPC
Class: |
H01S 5/06837 20130101;
H01S 5/0687 20130101; H01S 5/02438 20130101; H01S 5/0014 20130101;
G01J 1/4257 20130101; H01S 3/1303 20130101 |
Class at
Publication: |
372/056 ;
250/201.1 |
International
Class: |
H01S 3/22 20060101
H01S003/22; G01J 1/20 20060101 G01J001/20 |
Claims
1. A tracking system comprising: (A) a tracking unit comprising an
excited state atomic line filter comprising a metal vapor cell
having an optical entrance port and an optical exit port and
containing a metal vapor defining a first excited energy state,
defining a first resonant frequency, and a second excited energy
state and having at least one absorption line, at or near a desired
filter wavelength within an infrared spectral range, said
absorption line being equal, in energy, to a difference between the
second excited state and the first excited state and (B) a beacon
laser system, located on a platform to be tracked by said tracking
unit, for producing a beacon laser beam at a wavelength within said
narrow spectral band.
2. The tracking system as in claim 1 wherein said excited state
atomic line filter also comprises a pump laser for pumping said
metal vapor cell.
3. The tracking system as in claim 2 wherein said pump laser is a
diode laser.
4. The tracking system as in claim 3 wherein said excited state
atomic line filter further comprises two frequency locking etalons
and two frequency locking detectors for monitoring frequency of
said pump beam.
5. The filter as in claim 4 wherein said two etalons are tuned to
produce transmission peaks on both sides of said first resonant
frequency and feedback control to maintain said diode laser
operation at said resonant frequency based on input signals from
said two frequency locking detectors.
6. A tracking system as in claim 1 wherein said beacon laser system
comprises: A) a diode laser, B) two frequency locking etalons and
two frequency locking detectors for monitoring frequency of said
beacon laser beam and C) feedback control components to maintain
said wavelength of said beacon laser beam within said narrow
spectral band of the excited state atomic line filter.
7. A tracking system as in claim 5 wherein said beacon laser system
comprises: A) a diode laser, B) two frequency locking etalons and
two frequency locking detectors for monitoring frequency of said
beacon laser beam and C) feedback control components to maintain
said wavelength of said beacon laser beam within said narrow
spectral band of the excited state atomic line filter.
8. The tracking system as in claim 7 wherein said two etalons are
tuned to produce transmission peaks on both sides of said first
resonant frequency and feedback control to maintain said diode
laser operation at said first resonant frequency based on input
signals from said two frequency locking detectors.
9. The tracking system as in claim 1 wherein said beacon laser is
configured to produce a beacon laser beam with a divergence of at
least 20 degrees.
10. The tracking system as in claim 8 wherein said beacon laser is
configured to produce a beacon laser beam with a divergence of at
least 20 degrees.
11. A tracking system comprising: A) an atomic line filter
comprising: 1) a metal vapor cell having an optical entrance port
and an optical exit port and containing a metal vapor defining a
first excited energy state, defining a first resonant frequency,
and a second excited energy state and having at least one
absorption line, at or near a desired filter wavelength within an
infrared spectral range, said absorption line being equal, in
energy, to a difference between the second excited state and the
first excited state; 2) at least one magnet for imposing on said
metal vapor a magnetic field to produce polarization rotation near
said at least one absorption line; 3) a first polarizer positioned
to block polarized light at a first polarization from entering said
vapor cell through said entrance port; 4) a second polarizer
oriented perpendicular to said first polarization and located
across the path of light exiting said exit port so as to block
light exiting said vapor cell that is polarized perpendicular to
said first polarization; 5) a pump light source for producing light
at said first resonant frequency for pumping said metal vapor from
said ground state to said first excited state; wherein said metal
vapor under the influence of said magnetic field produces a
polarization rotation of light within a narrow spectral band near
said absorption line permitting light within this spectral band to
pass through said second polarizer whereas all light in a much
wider spectral range is not rotated in polarization and is blocked
by either the first polarizer or the second polarizer; B) a beacon
laser located on a platform to be tracked by said tracking unit,
for producing a beacon laser beam at a wavelength within said
narrow spectral band, said beacon laser system comprising: 1) a
diode laser, 2) two frequency locking etalons and two frequency
locking detectors for monitoring frequency of said beacon laser
beam and 3) feedback control components to maintain said wavelength
of said beacon laser beam within said narrow spectral band.
12. The tracking system as in claim 11 wherein said pump light
source in said excited state atomic line filter is a pump laser
system.
13. The tracking system as in claim 12 wherein said pump laser
system is a diode laser system producing a pump beam
14. The tracking system as in claim 13 wherein said diode laser
further comprises two frequency locking etalons and two frequency
detectors for monitoring frequency of said pump beam.
15. The tracking system as in claim 14 wherein said two etalons are
tuned to produce transmission peaks on both sides of said first
resonant frequency and feedback control to maintain said diode
laser operation at said resonant frequency based on input signals
from said two frequency locking detectors.
16. The tracking system as in claim 15 wherein said metal vapor is
an alkali metal.
17. The tracking system as in claim 15 wherein said metal vapor is
rubidium.
18. The tracking system as in claim 17 wherein at least one magnet
is two permanent magnets.
19. The tracking system as in claim 18 wherein said two permanent
magnets are two ring magnets.
20. The tracking system as in claim 11 wherein said at least one
magnet comprises an electromagnet.
21. The tracking system as in claim 16 wherein said vapor cell
defines beam direction and at least one magnet is oriented to
produce a magnetic field parallel to said beam direction.
22. The tracking system as in claim 16 wherein said vapor cell
defines a beam direction and said at least one magnetic is oriented
to produce a magnetic field perpendicular to said beam direction.
Description
[0001] The present invention related to tracking systems and
optical filters, especially to atomic line filters (ALF). This
application is a continuation in part of Ser. No. 10/682,567 soon
to issue as U.S. Pat. No. 7,058,110.
BACKGROUND OF THE INVENTION
[0002] Atomic line filters are a class of optical filters which
have acceptance bandwidths on the order of 0.001 nm. In one prior
art type of ALF, broadband light containing narrowband signal light
is passed through a first color glass filter which cuts off light
at wavelengths below a threshold value. The signal and remaining
noise light enter an atomic vapor that only absorbs the signal
light within the atom's 0.001 nm acceptance bandwidth thereby
exciting those absorbing atoms to an intermediate energy level. A
pump beam further excites those atoms to a second, higher energy
level that then decays through various processes including
fluorescence, to the ground state of the atom. The emitted
fluorescence occurs at wavelengths below the threshold value of the
first color glass filter. A second color glass filter then cuts off
any wavelengths above the threshold which effectively permits
passage of only the emitted narrowband fluorescence. In effect, the
incoming signal has been internally shifted in wavelength by the
atomic vapor, which then allows the use of two overlapping color
glass filters to block any background radiation.
[0003] Another type of prior art atomic line filter takes advantage
of either the Faraday effect or the Voigt effect where an atomic
vapor in a magnetic field produces polarization rotation in order
to pass a narrow spectral band of light through two crossed
polarizers. These filters are known respectively as Faraday filters
or Voigt filters. An important use for these filters is to block
background light so that a beacon laser beam can be detected by a
wide field-of-view detector.
[0004] Operational principles of Faraday filters can be understood
by reference to FIGS. 7A-C. Crossed polarizers 90 and 91 serve to
block out background light with a rejection ratio better than
10.sup.-5. Because these polarizers only work over a limited
wavelength region in the infrared, a broad band interference filter
may be used in conjunction with the Faraday filter. Between the
polarizers, an atomic vapor (which in many of these filters is
cesium or rubidium) in a magnetic field axially aligned with the
path of the beam, rotates the polarization of the laser signal by
90.degree., while leaving background light at other wavelengths
unrotated, and thus blocked by the polarizers.
[0005] In the case of the Faraday filter the magnetic field is
applied in the direction of the signal beam, and in the case of the
Voigt filter the magnetic field is applied perpendicular to the
signal beam direction and at 45 degrees to the direction of each of
the two cross polarizers.
[0006] Prior art atomic line filters patents issued to co-workers
of applicant includes U.S. Pat. Nos. 4,983,844; 5,267,010;
5,502,558; 5,731,585 and 6,151,340 each of which are incorporated
herein by reference. The '844 patent discloses a fast atomic line
filter which utilizes a pump laser and a high voltage potential to
produce ion pairs from atoms excited by photons with wavelengths
corresponding to a resonant frequency. The other patents describe
applications of Faraday and/or Voigt filters.
[0007] One problem with atomic line filters such as those referred
to above is that their operation depends on the existence of a good
sharp resonant absorption line near the spectral range to be
filtered. Many of these sharp resonant absorption lines are
characteristic of atomic vapors and the filters described in the
above referenced patents utilize alkali metals such as cesium and
rubidium to produce these metal vapors. These metals are preferred
because their vapors may be produced at relatively low
temperatures. However, good absorption lines from these alkali
metal vapors are generally in the visible and the near visible
spectral region such as 780 nm and 852 nm.
[0008] Many optical symptoms operate at wavelengths substantially
longer than the visible and near visible. A good example is light
with wavelengths in the range of 1.5 micron. For example, fiber
optic communication is typically at wavelengths in the range of
about 1.2 micron to about 1.65 microns (see FIG. 5). Typical long
haul filter optics operate within a C or L band. (C band is 1520 nm
to 1570 nm, and L band is 1570 nm to 1620 nm). Shorter range fiber
optics requiring higher quality fiber optics may operate within an
s band (1.31-1.48 microns). Also, there is a need for filters at
even longer wavelength such as about 1.5 to 5 microns for laser
tracking and for free space laser communications. Applicants have
searched for good resonance lines in the alkali metals at these
wavelengths without success.
SUMMARY OF THE INVENTION
[0009] The present invention provides a tracking system utilizing
an excited state atomic line filter. The filter includes a metal
vapor cell having an optical entrance port and an optical exit port
and containing a metal vapor having a first excited energy state
with a resonant frequency, and a second excited energy state. The
cell has an absorption line, at or near a desired filter
wavelength. The platform to be tracked, which could be an un-manned
aerial vehicle has a beacon laser system located on it for
producing a beacon laser beam at a wavelength within the narrow
spectral band. The present invention solves the problem of lack of
ground state resonant lines in at wavelengths substantially longer
than those of visible light. Atomic line filters of the Faraday or
Voigt crossed polarizer type are provided in which alkali metal
atomic vapor in a vapor cell is excited with a pump beam to an
intermediate excited state where a resonant absorption line, at a
desired wavelength, is available. A magnetic field is applied to
the cell producing a polarization rotation for polarized light at
wavelengths near the resonant absorption lines. Thus all light is
blocked by the cross polarizers except light near one of the spaced
apart resonant lines. However, the polarization of light at certain
wavelengths near the resonant is rotated in the cell and therefore
passes through the output polarizer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1A shows a laboratory demonstration of the present
invention with a cesium cell.
[0011] FIG. 1B shows a laboratory demonstration of the present
invention with a rubidium cell.
[0012] FIG. 2 is a transmission curve at approximately 1469.49 nm
with the cesium cell.
[0013] FIG. 3 is a drawing of a preferred embodiment of the present
invention.
[0014] FIG. 3A shows wavelengths locking features using dual
etalons.
[0015] FIG. 3B shows wavelengths locking features using dual
etalons.
[0016] FIG. 3C shows wavelengths locking features using dual
etalons.
[0017] FIG. 3D shows wavelengths locking features using dual
etalons.
[0018] FIG. 3E shows expected transmission curve for the FIG. 3
embodiment.
[0019] FIG. 4 shows actual transmission curve at 1529.3 nm for the
rubidium cell laboratory demonstration.
[0020] FIG. 5 shows wavelength bands in wide use for fiber optic
communication.
[0021] FIG. 6A shows some atomic transmission for cesium and
rubidium.
[0022] FIG. 6B shows some atomic transmission for cesium and
rubidium.
[0023] FIG. 7A shows features of a prior art Faraday filter.
[0024] FIG. 7B shows features of a prior art Faraday filter.
[0025] FIG. 7C shows features of a prior art Faraday filter.
[0026] FIG. 8 shows background spectral data.
[0027] FIG. 8A show prior art Faraday and Voigt results.
[0028] FIG. 8B shows prior art Faraday and Voigt results.
[0029] FIG. 8C shows prior art Faraday and Voigt results.
[0030] FIG. 9 shows an application of the present invention.
[0031] FIG. 10A shows etalon data.
[0032] FIG. 10B shows etalon data.
[0033] FIG. 10C shows etalon data.
[0034] FIG. 10D shows a laboratory demonstration of a signal laser
locking technique.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Prototype Results
Cesium Vapor Cell
[0035] FIG. 1 shows a prototype layout used by Applicants to
demonstrate the present invention. For this demonstration, a 10 mm
long cesium cell, 4 containing cesium vapor at 157.degree. C. is
placed in a magnetic field of 150 Gauss produced by two ring
magnets, 6 (Dexter Permag PP96-1213-1). The cesium vapor was
excited with a 852 nm pump beam to the Cs 6.sup.2P.sub.3/2 level as
shown in FIG. 3 by pump laser 8 which is a diode laser supplied by
SDL (now part of JDS Uniphase) (SDL 5722-H1). For this
demonstration a tunable external cavity diode laser 10 (Near Focus
Velocity 6326) is used for the signal beam so that a wide spectral
range could be examined. The laser is tunable within the range of
1446-1557 nm. The output beam is linearly polarized. A long pass
filter 12 blocks virtually all wavelengths below about 1.0 .mu.m
including light at the pump beam wavelength of 852 nm and Wallaston
polarizing prism 14 is arranged with its polarization direction
perpendicular to the polarization direction of the signal beam from
tunable laser 10. Neutral density filter 16 reduces the intensity
of light across a broad spectrum to match the intensity of the
signal light operating range of the test instruments. Thus,
substantially all light is blocked except light from laser 10 which
is polarization rotated within cesium cell 4.
[0036] FIG. 2 shows the results obtained by scanning the laser
through the range of about 0.05 nm centered at 1469 nm. As is
evident from the figure almost 100% of the light with a wavelength
band of less than 0.01 nm is transmitted with substantially zero
transmission outside a band of about 0.05 nm. The two side lobes
shown in FIG. 2 would produce some noise but in many application
this would be insignificant.
Rubidium Vapor
[0037] A second bench top experiment was conducted using a rubidium
cell. This experiment was very similar to the cesium cell
experiment and is shown in FIG. 1B. The only difference is that the
vapor cell 4 is a rubidium vapor cell and the pump laser is a 780
nm pump laser model Vortex 6013 supplied by New Focus with offices
in San Jose, Calif. and the half wave plate 9 is chosen to
correspond to the 780 nm wavelength. The results were equivalent to
those shown for the co-experiment. This experiment shows that an
atomic line filter can be provided in accordance with the present
invention to provide an excellent filter for the very important C
band of fiber optics communication. See FIG. 5.
Wavelength Locking
[0038] To fully utilize the excited atomic line filters of the
present invention, the signal laser frequency must be stabilized
within the transmission bandwidth of the filters during operation
which may encounter temperature drift, vibration and mechanical
shock. Efforts have been put forwards to stabilize the laser
frequency, the success, however, is limited up to date. Earlier
wavelength locker design described in prior art patents cited in
the Background section utilized an extended cavity laser with an
ALF inside the cavity. Applicants have discovered that this design
is sensitive to vibration and mechanical shock. The laser is
scanned in frequency to locate the peak ALF transmission position.
The laser is then periodically scanned and tuned to keep the
frequency at the peak transmission position. Additional
complications results from the fact that the laser frequency is
very sensitive to the laser diode temperature which is affected not
only by the environmental temperature, but also by the tuning of
the laser current.
[0039] Preferred embodiments of the present invention employ a pair
of micro-etalons in order to provide frequency locking of the
signal laser. Preferably the transmission peaks of the etalons
differ by the full width at the half maximum (FWHM) of the
transmission band of the atomic line filer and the peak frequency
of one of the etalon is located at one side of the filter
transmission band and the peak frequency of the other etalon is
located at the other side of the filter transmission band. The
laser wavelength is locked at the middle of the pair of the
transmission bands. FIG. 10A shows the simulated etalon
transmission spectra of a pair of etalons by CVI Technical Optics.
The peak transmission of each of the etalons can be adjusted by
changing the etalons spacing and/or charging the angular position
of the etalon relative to the sample beam as shown in FIGS. 10B and
10C. Etalon performance is also affected by temperature and
pressure changes.
[0040] Several parameters need to be considered in designing an
etalon:
a) Transmission Wavelength and Free Spectra Range
[0041] An etalon is compose of two highly reflective parallel
surfaces with the medium between the two surfaces either air or
solid. Etalon transmissions are results of multiple beam
interference. The transmitted wavelength is determined by
.lamda.=(2n'hcos .theta.')/m where, n' is the index of refraction
of the medium in the etalon gap; h is the thickness of the etalon
gap; .theta.' is the refraction angle inside the medium; and m is
the interference order. The transmission peaks are equally spaced
in frequency with frequency spacing, the so-called free spectra
range (FSR), determined by C/(2n'hcos .theta.'), where C is the
speed of light. The transmission wavelength for a given
interference order can be tuned by varying the angle of refraction,
or the spacing of the etalon, or index of refraction of the
material in the space between the surfaces that material is
typically a gas such as air and n is affected by the pressure and
temperature of the gas. FIGS. 10B and 10C plot the frequency shift
of the transmission band with angle of refraction and etalon
spacing, respectively, for an air-spaced etalon with 7.5 mm spacing
or 20 GHz FSR at 852 nm central transmission wavelength. To achieve
sub 100 MHz resolution angle-tuning of the transmission band, the
required angular control accuracy is on the order of hundreds of
micro radian. b) FWHM and Finesse [0042] The finesse (f) of the
etalon defined as the FSR divided by the FWHM of the transmission
bands is only determined by the reflectance (R) of the etalon f =
.pi. .times. R 1 - R ##EQU1## [0043] The larger the reflectance,
the larger the finesse and therefore narrower transmission
bandwidth for a given spacing or FSR. To produce a FWHM of 0.5 GHz
at 780 nm with a FSR of 25 GHz, the required reflectance is about
94%. In practice, defects in Optics will deteriorate the finesse of
the etalons producing a somewhat wider transmission band. c)
Thermal Stability [0044] Etalons can be air-spaced or solid. For
better thermal performance, air-spaced etalons are the choice
because the spacers can be made of temperature insensitive
materials such as Zerodur. The cavity may also be sealed to reduce
index of refraction fluctuation with temperature. The quoted
thermal stability frm WavePrecision Inc. is -0.007 GHz/.degree. C.
for sealed air-spaced etalons made of Zerodur.
[0045] Preferred etalons are air-spaced with 7.690 mm spacing which
results in a free spectra range (FSR) of 19.5 GHz. The reflectivity
of the coating is 90% at 780 nm, which leads to a finesse of 31.
The corresponding FWHM of the transmission bands is about 0.7
GHz.
[0046] A schematic diagram of a wavelength locker for locking a
diode laser is shown in FIG. 10D. Mini etalons and micro optics are
used. The output from laser diode 70 is first collimated. A few
percent of the collimated beam will be then picked off by beam
splitter 72 for wavelength locking. The sampled beam will be split
into two beams, one of which will be sent to a small absorption
cell 74 of atomic vapor for absolute wavelength tuning of the laser
wavelength, another beam will be sent to the etalon pair assembly
76A and 76B for wavelength locking signals from detection D1, D2
and D3 are used for tuning the etalon on both sides of the
absorption peak. The whole package will be temperature-controlled
with a TE cooler. The reader should not that once etalons have been
tuned and locked in place, the absorption cell may no longer be
needed. Also, for locking the beacon laser, the atomic line filter
is preferably used for absolute tuning.
APPLICATIONS
Laser Tracking
[0047] A preferred application of the present invention is for
laser tracking of a moving target. In this case the moving target
may utilize a relatively wide divergence beacon laser which is
tracked by a tracking detector which includes one or more filters
such as one of those described above. The beacon laser system
should be stabilize the beacon laser at the specific wavelength
band of the very narrow band filter. In this case, a very narrow
band filter is needed so that reflected and scattered sunlight can
be filtered out. FIG. 8 shows examples of spectral background
looking directly at the sun 80, looking at a sunlit cloud 82,
scattering from a clear sky and emission from a clear sky. A
tracking system must compete with this light and must produce a
sufficient signal to noise ratio to stay on track. FIG. 9 is a
drawing showing ground-to-air tracking. The technique used is to
provide a beacon laser operating at or within a precise wavelength
range with a wide enough angle to be relatively certain of
including the tracking system.
[0048] This provides the tracking system with a narrowband filter
which will be as efficient as possible at the beacon laser
wavelength and as narrow enough so as to discriminate sufficiently
against background light so that the signal to noise ratio is large
enough to permit tracking.
[0049] Dielectric optical bandpass filters typically achieve
bandwidths of 0.5% to 1% of the center wavelength, and suffer from
increased transmission loss with decreasing bandwidth, limited
acceptance angle, and wavelength shift with temperature. Atomic
line filters have demonstrated superior performance with bandwidths
less than 0.02 nm, greater than 30 degree acceptance angle, and
little or no dependence upon temperature. However, these filters
are limited to wavelengths in the visible and near visible (<853
nm). A new excited state Faraday resonance filter of the present
invention extends conventional proven atomic line filter techniques
to the 1550 nm region. This filter provides increased background
reduction of >100 over the best optical filters without the
angular field of view and temperature shift limitations of
dielectric optical filters.
[0050] For negligible electronic noise, an FPA used in beacon
acquisition will have an SNR given by SNR = [ .eta. .times. .times.
P s 2 .times. hv .times. .times. .DELTA. .times. .times. f
.function. ( 1 + P b / P s ) ] 1 / 2 ##EQU2## where P.sub.s is the
per pixel detected signal power, P.sub.b is the per pixel
background power and .DELTA.f is the sensor integration bandwidth.
For background limited performance it can be shown that the IFOV
.theta. = ( P s / A .DELTA..lamda. .times. .times. L .lamda. ) 1 /
2 ##EQU3## when P.sub.s=P.sub.b, .DELTA..lamda. is the bandpass, A
is the pixel area, and L.sub..lamda. is the background radiance
(W/cm.sup.2/SR/.mu.). This important result shows that decreasing
the optical filter bandwidth by a factor of 100 will allow the
received beacon power density to be decreased by 100 for the same
sensor field of view and background power density. Or,
alternatively, will allow 100.times. more solid angle to be
illuminated thereby decreasing search time.
[0051] Dielectric bandpass filters having narrow bandwidths will
have center transmission wavelength shift with angle of incidence
(.theta.) proportional to sin.sup.2.theta.. For a SOTA filter of
0.1% bandwidth, this shift would exceed the bandwidth for an angle
of incidence of 10 degrees (FIG. 2). Thus the new ultra-narrow
filters being made for fiber DWDM systems (ie 0.1% BW) will not
meet the viewing angle requirements needed for an acquisition
sensor. The 1.55 micron atomic line filter described above can
readily achieve 1/100 of this bandwidth (0.02 nm 1550 nm) with
greater than 30 degree acceptance angle and 80% transmission.
PREFERRED EMBODIMENT
[0052] Details of the principal components and features of a
preferred embodiment of the present invention are shown in FIGS. 3,
3A, 3B, and 3C. This embodiment includes a beacon laser unit 30
that might be located on a moving platform such as an unmanned
aerial vehicle such as those shown in FIG. 9. It also includes
excited state atomic line filter unit 32 comprising rubidium ALF 34
and pump laser unit 36.
[0053] The beacon laser unit comprises diode signal laser 38,
frequency locking etalons 40A and 40B, and detectors 42A and 42B
providing frequency locking signals to processor 44 which utilizes
those signals to control the current to signal laser 38 through
current control 46. Processor 44 also maintains control of the
diode temperature and the chamber temperature through
thermoelectric coolers (not shown) and diode temperature control
unit 46 and chamber temperature control unit 48.
[0054] Pump laser unit 36 provides a laser beam locked precisely to
a wavelength .lamda.=780.027 nm to excite rubidium vapor in vapor
cell 70 to the 5.sup.2P.sub.3/2 excited state as shown in FIG. 6B.
Controls for this laser are similar to those for beacon laser 30.
The unit includes diode pump laser 50, frequency locking etalons
52A and 52B and detectors 54A and 54B providing frequency locking
signals to processor 56 which utilize these signals to control the
current to pump laser 50 through current control 58. Processor 56
also maintains control of the diode temperature and the chamber
temperature through thermoelectric coolers (not shown) and diode
temperature control unit 57 and chamber temperature control unit
59. The output beam 60 of pump laser unit 36 is co-aligned with
beacon beam 62 from beacon laser 30 using dichroic mirror 64.
[0055] Rubidium atomic line filter 34 comprises rubidium cell 70,
two ring magnets 72 producing a 150 Gauss co-axial magnetic field
through cell 70, cross polarizers 74 and 76, long pass filter 78
and detector 80. Temperature controller 82 controls the temperature
of cell 70 via rubidium heater 84. Cold finger 86 provides a
condensation location within the cell to maintain a rubidium
vapor-liquid equilibrium within cell 70. The signal from detector
as shown at 88 may be utilized by pointing equipment (not shown) to
maintain a telescope such as those shown in FIG. 9 pointed at
beacon laser 30.
[0056] FIG. 3A provides a qualitative representation of the output
of detection D1 and D2 of beacon signal laser 30 as a function of
the output wavelength of beacon signal laser 38. FIG. 3B shows
qualitatively the relationship between the ratios of the detector
signals to the wavelength within the narrow spectral range of about
1529.29 nm to about 1529.31 nm. Current to diode laser 38 is
adjusted to maintain the output of beacon laser 38 precisely within
the transmission band of filter unit 32 as shown in FIG. 3F.
[0057] FIGS. 3C and 3D are similar graphs demonstrating the control
of the wavelength of the pump laser beam from pump laser unit 36.
This wavelengths can also be controlled to a precession of about
.+-.0.0005 nm. Specification for pump 36 laser provide a response
of 0.0034 nm per milliamps permitting wavelength precision to about
.+-.0.0005 nm.
Alignment and Packaging Procedure
[0058] The alignment and packaging process for both the pump laser
and the beacon laser should be performed in a well controlled
environment so that the laser wavelength will be stable to about
100 MHz even without wavelength locking in a period of one hour or
so. At first all components other than the two etalons will be
aligned and fixed to positions either mechanically or using
thermal/UV curing epoxy. The two etalons will be attached to two
precession (100.mu. radian or better resolution) angular alignment
stages, respectively, by mechanical or vacuum means. The beacon
laser is scanned to obtain an ALF transmission spectrum and the
pump laser is scanned to obtain an absorption spectrum. For the
beacon laser, the two etalons will be angularly-tuned so that their
transmission spectra overlaps at the center of the ALF spectrum.
For the pump laser, the transmission spectrum of the two etalon
overlap at the center of the absorption spectrum. The etalons will
be fixed to positions with UV epoxy.
Control Electronics and Operation Procedure
[0059] A microprocessor will be used to control the laser current
and process the outputs from the two etalon-photodiodes. The diode
laser TE cooler, the atomic vapor cell heater, and the TE cooler
for the whole package may also be controlled with the
microprocessor.
[0060] To operate the wavelength locker, at first all temperature
controls should be stabilized with the laser current ramped up to
the operation value. The laser wavelength will then be locked to
the position by balancing the light output from the two etalons
through the tuning of the laser current with the
microprocessor.
Packaging
[0061] The etalons will be made of temperature-insensitive
materials and the whole package may be temperature-controlled as
well. So thermal stability should not be a critical issue. However,
as shown in FIG. 10B, the peak transmission frequency of the
etalons is very sensitive to angle of incident. To lock the laser
frequency better than a few hundreds of MHz, the incident beam
should be stabilized better than 1 mradian. To reduce spatial
jitter of the incident beam from the beacon laser to the wavelength
locker, the whole package should be made of small size components
with dimensions on the order of a few centimeters. Furthermore, the
beacon laser and the line locker should be integrated into one
small package.
[0062] While the above description describes specific preferred
embodiment of the present invention, person skilled in this art
should understand that many changes and modifications could be made
within the scope of the present invention. For example, vapors
other than rubidium and cesium could be used. Specifically, other
alkali metals are good choices, where a desirable resonant
frequency exists between two excited states and where the vapor can
be pumped to the first excited state. Good application of the
present invention includes tracking a transceiver in a laser
communication system. Also, the system may be used to track hot
objects such as bullets or other missiles emitting radiation at
infrared wavelengths.
[0063] Another application is for laser radar systems where eye
safety is a concern. For these reasons the reader should determine
the scope of the invention from the appended claims and their legal
equivalence.
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